Although biometry, also referred to as biometrics, is generally concerned with measurements and the measuring and evaluation processes necessary for this purpose in organisms, in the following discussion it is not limited strictly to eyes.
A number of methods and measuring devices are known in the prior art for determining the known features of the structures of an eye; in this regard, primarily ultrasonic measuring devices and optical measuring devices based on short coherent interferometric methods or confocal scanners have become established. Of the numerous approaches known from the prior art, the medical-diagnostic importance of the mentioned measurements is evident.
A number of methods and measuring devices are known in the prior art for determining the known features of the structures of an eye; in this regard, primarily ultrasonic measuring devices and optical measuring devices based on short coherent interferometric methods or confocal scanners have become established.
The specific disadvantages of ultrasonic devices are, on the one hand, lower resolution, and on the other hand, the need for direct contact with the eye, which always entails the risk of transmission of infections, and also requires that the eye be anesthetized for the measured value determination. In ultrasonic devices there is no automatic alignment of the measuring beam on the visual axis of the eye, so that the likelihood of erroneous measurements is accordingly high.
Analogously to the ultrasonic devices, in which images of the structural transitions are reconstructed based on the acoustic signals, in the optical measuring devices based on short coherent interferometric methods, optical images of the structural transitions are represented as one-dimensional depth profiles (A scans) or two-dimensional depth section images (B scans). As a short coherent measuring method, the so-called optical coherence tomography (OCT) method has become established, in which coherent light is used for distance measurement of reflective and scattering materials with the aid of an interferometer. For depth scanning, optical coherence tomography for the human eye provides signal responses which are measurable at optical boundary surfaces due to changes in the refractive index.
The basic principle of the OCT method described in U.S. Pat. No. 5,321,501 A, for example, is based on white light interferometry, and compares the propagation time of a signal, using an interferometer (usually a Michelson or Mach-Zehnder interferometer). The arm having a known optical path length (also referred to as the reference arm) is used as a reference with respect to the measuring arm. The interference of the signals from the two arms produces a pattern from which the relative optical path length within an A scan (individual depth signal) may be read. In the one-dimensional raster process, the beam, analogously to ultrasonic technology, is then guided transversally in one or two directions, by means of which a flat B or C scan or a three-dimensional tomogram may be recorded.
In the OCT method used in ophthalmology, two different types have become established. For determining the measured values, in the first type the length of the reference arm is changed, and the intensity of the interference is continuously measured without taking the spectrum into account. This method is referred to as the “time domain” method. In contrast, in the other method, referred to as the “frequency domain” method, for determining the measured values the spectrum is taken into account and the interference of the individual spectral components is detected. Therefore, reference is made on the one hand to the signal in the time domain, and on the other hand, to the signal in the frequency domain. The advantage of the “frequency domain” method lies in the simple and rapid simultaneous measurement, in which complete information concerning the depth may be determined without necessarily requiring moving parts. This increases the stability and the speed. Due to the Fourier transform used for reconstructing the position information, these methods are also referred to as “Fourier domain” methods.
The “frequency domain” method may be divided into simultaneous methods and sequential methods, depending on the light source used. The simultaneous method, which requires a broadband light source such as a superluminescent diode (SLD) or a femtosecond laser, is also referred to as a “(parallel) spectral domain” method. In contrast, in the sequential method a tunable light source having a variable wavelength is used, the sequential “frequency domain” method also being referred to as the “swept source” (SS OCT) method. Common “swept source” light sources are tunable lasers which use rapidly variable spectral filters such as the Fabry-Perot filter, or wavelength selectors based on rotating polygon scanners, or also current-tunable semiconductor lasers. The tuning rates may be in the range of several hundred hertz to several megahertz.
In contrast, the “time domain” method may be divided into simultaneous methods and sequential methods, depending on the detector used, a broadband light source always being used. Whereas in the simultaneous “time domain” method the expanded measuring beam strikes a diode, CCD array, or CMOS array (full-field OCT), in the sequential “time domain” method the measuring beam is deflected onto a simple high-sensitivity photodiode via an interferometric beam splitter and a displaceable mirror in the reference arm. When an OCT scan is recorded at a constant setting of the reference arm, this is referred to as the performance of a C scan or an “enface OCT.” However, the term “enface OCT” is also sometimes used for the frontal views obtained from OCT volume scans.
The major technological advantage of OCT is the decoupling of the depth resolution from the transversal resolution. The depth resolution is determined only by the utilized bandwidth of the light source used. Common bandwidths are in the range of several nanometers to over one hundred nanometers, and when measuring radiation in the near infrared is used, 700-1350 nm. The depth resolutions thus achievable are in the range of 3-100 μm. In contrast to microscopy, the three-dimensional structure of the object to be examined may thus be detected, even when the numerical aperture, for example for small pupils in nondilated eyes, is greatly limited.
The purely reflective, and therefore contactless, measurement allows the generation of microscopic images of living tissue (in vivo). The wavelength of the measuring radiation to be used is determined by the desired application, taking into account the wavelength-dependent tissue absorption and back-scattering. If the ocular fundus, for example, is to be measured, in particular radiation in the range of 690-900 nm or 960-1100 nm is suited, and for the anterior portion of the eye, for example radiation in the range of 1260-1360 nm is suited.
The approach for eye diagnosis described in U.S. Pat. No. 5,347,328 A is based on the interferometric measurement of the length of the optical axis of an eye. For this purpose, the eye is illuminated with a coherent light beam whose wavelength is varied in a predetermined range. The change in the wavelength causes a change in the phase difference of the beams reflected on the boundary surfaces, which is used for determining the distance between the corneal surface and the ocular fundus.
The publication [1] by A. F. Fercher et al. describes the Fourier optical OCT method in general, and publication [2] also describes the specialized determination of the coherence function of the light reflected from the eye by inverse Fourier transformation of the spectral intensity distribution.
Use of the Fourier transform method in particular for measuring intraocular distances along a single beam through the pupil has been described by A. F. Fercher et al. in publication [3], and used by G. Häusler and M. W. Lindner according to publication [4] for producing OCT images.
DE 43 09 056 A1 describes a method for determining the distance and scattering intensity of scattering points, in which the distance and the local scattering intensity are determined by Fourier transformation of the spectrum according to the wavelength.
A method in which three-dimensional images of the retina may be synthesized from enface OCT recordings has been described by A. G. Podoleanu, J. A. Rogers, D. A. Jackson, and S. Dunne in publication [5].
A parallel OCT method which likewise uses a stepped reference mirror is described in U.S. Pat. No. 6,268,921 B1. The stepped reference mirror is used to achieve the depth scan in so-called time domain OCT. Accordingly, the step increments are much greater than λ/8. In addition, the steps are distributed not with periodically recurring overall heights, but, rather, over the entire surface in a stepped manner. The phase shifter which is also used in this approach acts equally on the entire reference arm or measuring arm. These differences naturally result from the other statement of the object also contained in the cited document.
A similar method based on piezoelectric phase shifting phase measurement is the subject of U.S. Pat. No. 6,377,349 B1. In this approach, the reference mirror is piezoelectrically displaced. However, this displacement and the necessary additional illumination, as well as the multiple readout of the photodetector array, are time-consuming, which results in motion artifacts in living objects such as the eye.
A conventional OCT method for determining the dimensions of the anterior portions of the eye, using a slit lamp and a hand-held device, has been described by S. Radhakrishnan et al. in publication [6]. The device, which is based on time domain OCT, operates very quickly, delivering 8 images per second. For example, for a three-dimensional representation of the anterior eye structure, the 8 images per second may be distributed equidistantly over the entire pupil, in which case approximately 1 second is required for the data recording.
In the optical measuring devices based on short coherent methods, the interferometer principle according to the dual beam method is also used. This method is characterized in particular by insensitivity to axial eye movements, since use is made of an interference between the light components reflected from the cornea and back-scattered by other eye structures. Approaches based on this measuring principle are described, for example, in DE 198 12 297 C2, DE 103 60 570 A1, and WO 2004/071286 A1.
As a result of the OCT methods as well as the noninterferometric confocal methods (US 2006/0158655 A1), accurate values of axial distances in optical path lengths are obtained. While the deviations in the OCT methods are less than the coherence length, the deviations in the confocal methods, depending on the quality of the scattered light suppression, are somewhat less favorable, but likewise are still in the submicron range. In the OCT methods it is particularly advantageous that interferometric measurements of optical path lengths may be carried out using very precise and stable external references, in particular also by measuring reference structures, or using reference interferometers for implementing a so-called k-clock [7].
One of the authors of the present patent application has previously published on various devices and methods for achieving improved, more stable signal strengths in the various regions of the eye, the contents of which are referenced below. This relates in particular to the approaches for improved performance of fixing marks (according to DE 10 2009 007732 A1), for changing focus positions (according to WO 2010/017954 A1), and for adapting reference planes (according to DE 10 2008 063225 A1).
Reproducible distance measurements having sufficiently good resolution and signal strength of media in the eye, for example corneal boundary surfaces or retinal layers, may be ensured by these methods. The refractive indices of the optical media, such as the cornea, the aqueous fluid, the lens, and the vitreous body, are sufficiently well known, and are defined, for example, in the Gullstrand eye model.
Furthermore, reference is likewise made to the two patents DE 101 08 797 A1 and EP 1 941 456 B1, relating to the automatic scanning evaluation of interferometric measurements of the eye for distance determination in eye structures.
In the approaches known from the prior art, section images of the media of the eye are generated by multiple, successively applied so-called depth scans, resulting in 3D representations. Depth scans or A scans for generating section images of the eye provide exact measured values, regardless of whether the scan is performed centrally through the pupil or at the pupil margin. Depending on the orientation of the eye, the scans may be performed in the direction of the optical axis, the visual axis, or any other given axis of the eye. The determined optical path lengths are converted into path lengths in the medium by means of the group refractive index of the particular optical media, taking into account the wavelength of the measuring radiation used.
A problem with the known approaches is that automatic evaluations of A and B scans for collecting biometric data are confronted with a number of measuring situations and interferences, in spite of which the automatic evaluations must still function accurately and with a minimum number of defects. Examples are eye length measurements in preparation for IOL implants in the treatment of cataracts, or severe refractive errors, or the replacement of IOLs.
In these cases, very different measuring conditions are present under which automatic scanning evaluation and collection of biometric data from OCT scans must reliably function, such as measurement radiation attenuation in cataracts, or measurement radiation defocusing in the case of refractive error, or also the presence of pathologies such as retinal edema. In the prior art this has been achievable only to a very limited extent, for which reason measured value deviations or incomplete measurement evaluations occur, which then require manual corrections of the distance measurements, which themselves may likewise contain errors.